1. Field of the Invention
The present invention relates to solid state lasers. More specifically, the present invention relates to systems and methods for atmospheric sensing using solid state lasers.
2. Description of the Related Art
Lasers are currently widely used for communication, research and development, manufacturing, directed energy and numerous other applications. For many applications, the energy efficiency, power and lightweight of solid state lasers makes these devices particularly useful. Solid state lasers currently lase in the range of one to three microns.
For certain applications, there is a need to reach higher laser operating frequencies. In particular, there is interest in the 8-12 micron (xcexcm) region. The 8-12 micron region provides an xe2x80x98open windowxe2x80x99 to the atmosphere making it useful for many applications. The window is xe2x80x98openxe2x80x99 in the sense that there is little atmospheric attenuation of the energy in the beam in this region of the electromagnetic spectrum. Hence, the 8-12 micron window allows for a probing of the atmosphere.
One such application, for which there is a need to probe the atmosphere, is that of remote sensing of chemical agents. Remote detection of toxic chemical agents is of current interest to both military and civilian defense agencies due to the growing availability and use of these compounds by terrorist groups and rogue nations. The 8-12 xcexcm spectral region of the atmosphere offers an opportunity to remotely detect commonly used chemical agents since these species typically have distinct band structure in this wavelength range, and there is relatively low atmospheric attenuation in this region.
Wavelength conversion to this region has been demonstrated using various solid-state lasers, or with optical parametric oscillators (OPOs) as pump sources for longer wavelength OPOs and difference frequency generation crystals. See for example: 1) S. Chandra, T. H. Allik, G. Catella, R. Utano, J. A. Hutchinson, xe2x80x9cContinuously tunable 6-14 xcexcm silver gallium selenide optical parametric oscillator pumped at 1.57 xcexcm,xe2x80x9d Appl. Phys. Lett. 71, 584-586 (1997): 2) T AIlik, S. Chandra. D. M. Rines, P. G. Schunemann, J. A. Hutchinson, and R. Utano, xe2x80x9c7-12 xcexcm generation using a Cr, Er:YSGG pump laser and CdSe and ZnGeP2 OPOs,xe2x80x9d in Advanced Solid State Lasers, OSA Trends in Optics and Photonics (Optical Society of America, Washington, D.C., 1997), Vol. 10, pp. 265-266; and 3) R. Utano and M. J. Ferry, in Advanced Solid State Lasers, OSA Trends in Optics and Photonics (Optical Society of America, Washington, D.C., 1997), Vol. 10, pp. 267-269[WJB1].
These approaches generally involve the use of a flashlamp pumped (Cr, Er:YSGG) laser emitting at 2.79 microns to pump a cadmium-selenide (CdSe) laser. This method has been represented as being effective to yield a tunable 8-12 micron output. Unfortunately, the laser is too large and inefficient to be feasible in the field. That is, the poor overall electrical efficiency of the Cr, Er:YSGG pump laser, together with its fairly long (50 ns) output pulse width, result in a less than optimal CdSe OPO pump source.
On the other hand, carbon-dioxide (CO2) lasers lase at 10 microns. However, these devices are not tunable and not sufficiently portable to be feasible for use in the field.
Hence, a need remains in the art for an efficient, feasible, portable, tunable system or method for converting the output of a typical 1-3 xcexcm laser to the 8-12 xcexcm range.
The need in the art is addressed by the system and method of the present invention. The system includes a transmitter having a laser for providing a collimated beam of electromagnetic energy at a first frequency and a Q switch in optical alignment with the beam. The system further includes a crystal for shifting the frequency of the beam from the first frequency to a second frequency. A mechanism is included for shifting the beam from the second frequency to a third frequency.
In the particular implementation, the third frequency is in the range of 8-12 microns. Ideally, the input beam is provided by a neodymium-YAG laser and the Q switch is a passive Q switch. The crystal is x-cut potassium titanyl arsenate.
In the best mode, the system includes a mechanism for switching the polarization state of the second beam and providing third and fourth beams therefrom. The third beam has a first polarization and the fourth beam has a second polarization. The second polarization is orthogonal relative to the first polarization. The mechanism for shifting the beam from the second frequency to the third frequency includes first and second optical parametric oscillators, each optical parametric oscillator including a cadmium selenide crystal. The frequency shifted third and fourth beams are combined to provide an output beam in the range of 8-12 microns. The output beam is transmitted and a return signal therefrom is detected by a receiver in the illustrative chemical sensing application.